Not World War II, but World War I:

The 1st Edition of Abraham Fraenkel’s book *Einleitung in die Mengenlehre* (*Introduction to Set Theory*) went to press during World War I. Fraenkel had teached set theory to his comrades while being at war, and this book were his lecture notes, so to say. He also gave his *venia legendi* lecture during the war while being on furlough.